Device-to-device (D2D) subframe with cell identifier

ABSTRACT

Device-to-device (D2D) user equipment (UE) devices transmit D2D subframes that include a cell identifier of the base station providing wireless service to the D2D UE device. A D2D UE device requesting D2D communication resources for D2D communication with other D2D UE device(s) indicates the cell identifiers of the cells serving the other D2D device(s) to the base station serving the requesting D2D UE device.

PRIORITY CLAIM

The present application claims priority to Provisional Application No.61/919,284 entitled “Scheduling for D2D Communications,” filed Dec. 20,2013, assigned to the assignee hereof and hereby expressly incorporatedby reference in its entirety.

FIELD

This invention generally relates to wireless communications and moreparticularly to transmission of cell identifiers in device-to-device(D2D) subframes.

BACKGROUND

Many wireless communication systems use base stations (communicationstations, transceiver stations, eNodeBs, eNBs) to provide geographicalservice areas where wireless communication user equipment devices (UEdevices) communicate with the base station providing the particulargeographical service area in which the UE devices are located. The basestations are connected within a network allowing communication links tobe made between the UE devices and other devices. The communicationsystem may include base stations that provide overlapping service areasof different sizes to serve various needs of mobile UE devices. Forexample, macrocell communications stations may provide macrocell serviceareas that are larger and may cover one or more small cell service areasprovided by small cell base stations.

In some circumstances, the communication links are between wirelesscommunication UE devices that are close to each other. In thesesituations, it may be preferred to have a direct communication linkbetween the two wireless UE devices rather than communicating through abase station. Such direct communication between devices is oftenreferred to as device-to-device (D2D) communication or peer-to-peer(P2P) communication. The communication resources (e.g., time frequencyblocks) used for D2D communication are typically a subset of thecommunication resources used by the communication system forcommunication between UE devices and the base stations.

An in-coverage (InC) UE device is a UE device that is within the servicearea of a base station and is capable of communication with the basestation. An out-of-coverage (OoC) UE device is a UE device that is notwithin a service area of any base station. D2D UE devices that areengaged in D2D communication with each other form a group. A D2D group,therefore, includes two or more D2D UE devices. There are five typicalcoverage scenarios that occur with D2D groups. In a first coveragescenario, all the UE devices of the D2D group are located in a servicearea of a single base station. For the first scenario, therefore, all ofthe D2D UE devices of the D2D group are InC UE devices in a singleservice area. In a second coverage scenario, none of the UE devices ofthe D2D group are located inside any service area of any base station.For the second scenario, therefore, all of the D2D UE devices of the D2Dgroup are InC UE devices. In a third coverage scenario, at least one ofthe UE devices of the D2D group is located in a service area of a singlebase station and at least one D2D UE device of the group is outside allservice areas. For the third scenario, therefore, at least one of theD2D UE devices of the D2D group is InC UE device in a single servicearea and at least one UE device is an OoC UE device. The third coveragescenario is sometimes referred to as a partial coverage scenario. In afourth coverage scenario, at least one of the UE devices of the D2Dgroup is located within a first service area of a first base station andat least one D2D UE device of the group located within a second servicearea of a second base station. For the fourth scenario, therefore, atleast two of the D2D UE devices of the D2D group are InC UE devices indifferent service areas. In a fifth coverage scenario, at least one ofthe UE devices of the D2D group is located within a first service areaof a first base station, at least one D2D UE device of the group locatedwithin a second service area of a second base station, and at least oneUE device of the D2D group is an OoC UE device. For the fifth scenario,therefore, at least two of the D2D UE devices of the D2D group are InCUE devices in different service areas and at least one D2D UE device ofthe group is an OoC UE device. Other coverage scenarios are possible.For the fourth and fifth coverage scenarios at least one UE device isserved by one base station and at least one UE device is served byanother base station. Such scenarios are typically referred to as aninter-cell coverage scenario. Since the UE devices of a group aremobile, there are situations where the coverage scenario of a D2D groupchanges. For example, a UE device of a D2D group may move from a firstservice area served by a first base station to a second service areaserved by a second base station.

SUMMARY

Device-to-device (D2D) user equipment (UE) devices transmit D2Dsubframes that include a cell identifier of the base station providingwireless service to the D2D UE device. A D2D UE device requesting D2Dcommunication resources for D2D communication with other D2D UEdevice(s) indicates the cell identifiers of the cells serving the otherD2D device(s) to the base station serving the requesting D2D UE device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a communication system where macrocellcommunication resources are used for device-to device (D2D)communication and user equipment (UE) devices transmit D2D subframesthat identify a serving cell.

FIG. 2A is a block diagram of a D2D subframe that identifies the servingcell of the UE device transmitting the D2D subframe where the cellidentifier is sent in the D2D message.

FIG. 2B is a block diagram of a D2D subframe that identifies the servingcell of the UE device transmitting the D2D subframe where the cellidentifier is embedded in the reference signal.

FIG. 3 is a block diagram of a wireless user equipment (UE) devicesuitable for use as the UE devices.

FIG. 4 is a block diagram of a base station suitable for use as the basestations.

FIG. 5 is a messaging diagram of an example of establishing inter-cellD2D communication.

FIG. 6A, is an illustration of the communication system when the firstUE device and the second UE device are in the first cell.

FIG. 6B is an illustration of the communication system during thehandover of the second D2D UE device to the second cell while the secondUE device is moving from the first cell to the second cell.

FIG. 6C, is an illustration of the communication system when handover iscomplete and the first UE device is in the first cell and the second UEdevice is in the second cell.

FIG. 7 is a message diagram where the second UE device is communicatingwith the first UE device and moves from the first cell to the secondcell.

DETAILED DESCRIPTION

A communication specification defines macrocell communication resourcesfor communication between base stations (communication stations, eNBs,etc.) and wireless communication user equipment (UE) devices. Themacrocell communication resources include defined downlink communicationresources and defined uplink communication resources, both defined bythe communication specification. Downlink communication resourcesselected from the defined downlink communication resources that areassigned (scheduled) for downlink communication are referred to asscheduled downlink communication resources. Uplink communicationresources selected from the defined uplink communication resources thatare assigned (scheduled) for uplink communication are referred to asscheduled uplink communication resources. In accordance with embodimentsof the invention, some macrocell communication resources are reservedfor device-to-device (D2D) communication. These reserved resources arereferred to as reserved D2D communication resources. Although defineddownlink communication resources and/or defined uplink communicationresources can be used for D2D communication, only defined uplinkresources are used for D2D communication in the examples herein. Aportion of the reserved D2D communication resources is reserved forout-of-coverage (OoC) D2D communication and another portion is reservedfor in-coverage (InC) D2D communication. The portion of D2Dcommunication resources reserved for out-of-coverage (OoC) D2Dcommunication is referred to herein as OoC D2D reserved communicationresources and the portion reserved for in-coverage (InC) D2Dcommunication is referred to herein as InC D2D reserved communicationresources. For the example, all reserved D2D communication resourcesthat are not reserved for OoC D2D communication are reserved for InC D2Dcommunication. As discussed below in further detail, different sets ofInC D2D reserved communication resources are assigned to different D2Dgroups. Communication resource allocation information indicating theresource assignment is broadcast to all of the D2D UE devices within theservice area of the base station in some circumstances. In othercircumstances, communication resource allocation information istransmitted to at least one of the D2D UE devices of the D2D group. TheInC D2D communication resources may be assigned to the D2D groups basedon a priority level of the group. The priority levels of the D2D groupscan be obtained from the network and/or relayed to the base station byone or more D2D UE devices. In some circumstances, a D2D group may be ina inter-cell coverage scenario where the D2D group includes at least oneD2D UE device in one cell and at least one other D2D UE device inanother cell. For the examples herein, D2D communication resources forsuch scenarios are managed by the base stations to identify acceptableD2D communication resources for the D2D communication between the D2D UEdevices in inter-cell D2D group. As discussed below, a D2D UE devicerequesting D2D communication resources for D2D communication with otherD2D UE devices served by other cells indicates, to the base stationserving the requesting D2D UE device, the cell identifiers of the cellsserving the other D2D devices that will receive D2D transmissions fromthe requesting UE device. The requesting D2D UE device determines thecell identifiers of the other cells from D2D subframes transmitted bythe other D2D UE devices. The D2D subframes, therefore, convey the cellidentifier of the serving cell of the device transmitting the D2Dsubframe. In some circumstances, the D2D subframes may be D2D discoverysubframes. Therefore, the cell identifiers may be transmitted in D2Ddiscovery signals.

FIG. 1 is a block diagram of a communication system 100 where macrocellcommunication resources are used for device-to device (D2D)communication. Base station 102, 104 provide wireless communicationservices to wireless communication UE devices 106, 108 withingeographical service areas 110, 112, sometimes referred to as cells.Several base stations are typically interconnected through a backhaul114 to provide several service areas to cover large areas. In somesituations, a base station may provide more than one cell. For example,a base station may include a single base station controller connected tomultiple radio heads to provide more than one cell. In the interest ofbrevity and clarity, the example of FIG. 1, each base station provides asingle cell. The techniques discussed herein may be applied to othercellular arrangements. A cellular communication system is typicallyrequired to adhere to a communication standard or specification. TheThird-Generation Partnership Project Long-Term Evolution (3GPP LTE)communication specification is a specification for systems where basestations (eNodeBs) provide service to wireless communication devices(user equipment (UE) devices) using orthogonal frequency-divisionmultiplexing (OFDM) on the downlink and single-carrierfrequency-division multiple access (SC-FDMA) on the uplink. Although thetechniques described herein may be applied in other types ofcommunication systems, the exemplary systems discussed herein operate inaccordance with a 3GPP LTE communication specification.

Each of the base stations 102, 104 is a fixed transceiver station,sometimes referred to as an eNodeB or eNB, which may include acontroller in some circumstances. Each base station may be connected toa network controller (not shown) within a cellular network through thebackhaul 114 which may include any combination of wired, optical, and/orwireless communication channels. For the examples herein, the controllerincludes the functionality of the Mobility Management Entity (MME) andthe Packet Gateway (P-GW). Accordingly, the controller typicallyincludes a scheduler which may schedule resources for communicationbetween the base stations and UE devices and, in some situations,between UE devices for D2D communication.

The wireless (UE) communication devices 106, 108 (collectively UEdevices) may be referred to as mobile devices, wireless devices,wireless communication devices, and mobile wireless devices, UEs, UEdevices as well as by other terms. The UE devices include electronicsand code for communicating with base stations and with other UE devicesin device-to-device configurations. The UE devices include devices suchas cell phones, personal digital assistants (PDAs), wireless modemcards, wireless modems, televisions with wireless communicationelectronics, and laptop and desktop computers as well as other devices.The combination of wireless communication electronics with an electronicdevice, therefore, may form a UE device. For example, a UE device mayinclude a wireless modem connected to an appliance, computer,television, or other device.

As explained above, in some situations, two or more UE devices maycommunicate directly with each other without communication through abase station. Such device-to-device (D2D) communication may occurbetween UE devices within the service area of a base station, outsidethe service area, or within multiple service areas. In FIG. 1, the firstD2D UE device (UE1) 106 is within the first service area (Cell 1) 110and the second D2D UE device (UE2) 108 is within the second service area(Cell 2) 112. In the example, the D2D UE devices 106, 108 establish D2Dcommunication using D2D communication resources assigned, or at leastallocated, by the base stations 102, 104.

The base station 102 includes a wireless transceiver (transmitter andreceiver) that can exchange wireless signals with the UE devices withinthe service area 110 such as the first D2D UE device 102. The secondbase station 104 also includes a wireless transceiver (transmitter andreceiver) that can exchange wireless signals with UE devices within theother service area (Cell 2) 112 such as the second D2D UE device 108.Transmissions from the base stations and from the UE devices 106, 108are governed by a communication specification that defines signaling,protocols, and parameters of the transmission. The communicationspecification may provide strict rules for communication and may alsoprovide general requirements where specific implementations may varywhile still adhering to the communication specification. Although thediscussion below is directed to the 3GPP Long Term Evolution (LTE)communication specification, other communication specifications may beused in some circumstances. The communication specification defines atleast a data channel and a control channel for uplink and downlinktransmissions and specifies at least some timing and frequencyparameters for physical downlink control channels from a base station toa UE device.

A substantial portion of the macrocell communication resources are usedfor transmitting the downlink signals and the uplink signals. Each ofthe base stations transmits downlink signals to the UE devices in itsrespects service area (cell) using scheduled downlink communicationresources of the defined downlink communication resources defined by thecommunication specification and reserved for downlink communication. TheUE devices transmit uplink signals to the base stations 102, 104 usingscheduled uplink communication resources of the defined uplinkcommunication resources defined by the communication specification andreserved for uplink communication. The macrocell communication resourcesinclude frequency bands divided in time where each frequency band andsegment of time can be identified by the scheduler and described incontrol signals sent from the base stations 102, 104 to the UE devices.The communication specifications, or other system rules, therefore,define applicable communication resources for the downlink andapplicable communication resources for the uplink. The schedulerallocates different time-frequency resources to different devices toefficiently utilize the resources while minimizing interference.Accordingly, the scheduled macrocell communication resources used forsignals exchanged with one UE device are different from scheduledmacrocell communication resources used for other signals exchanged withother UE devices. As referred to herein, therefore, the definedmacrocell communication resources are the communication resourcesreserved for communication by the specification and/or communicationsystem rules. The scheduled resources for macrocell transmission forparticular signals, however, are a subset of the reserved macrocellcommunication resources and are typically dynamically changed duringoperation.

For the examples herein, each D2D UE device transmits D2D subframes thatidentify the serving cell of the D2D UE device. Therefore, for theexample in FIG. 1, the first UE device 106 transmits D2D subframes 116that include a cell identifier that identifies the first cell 110.Although not shown in FIG. 1, the second UE device (UE2) 108 alsotransmits a discovery signal identifying the second cell 112. A suitabletechnique for transmitting the D2D subframe includes transmitting theD2D subframe in accordance with the Rel-8 PUSCH structure except thelast symbol in the D2D UL subframe is left blank. Is some situations theD2D subframe may be a D2D discovery subframe. The discovery signals aretransmitted in the resources assigned as a subset of discovery signalresource pool set.

As discussed below, the cell identifier may be conveyed by including aCell ID within the D2D subframe. The Cell ID is a unique (or at leastgeographically unique) identifier of a cell. For one example, the CellID is contained in the D2D subframe message. In another example, thescrambling embedded in the demodulation reference signal (DMRS) used fordemodulating the D2D subframe is manipulated to reflect the Cell ID. Forexample, the reference signal (RS) of a discovery signal can bescrambled using the Cell ID such that the other UE devices can determinethe Cell ID when descrambling the RS of the D2D subframes such asdiscovery signals.

If the second D2D UE device 108 determines that a D2D communication linkshould be established with the first D2D UE device 106, the second D2DUE device 108 transmits a D2D request 118 to its serving base stationwhere the D2D request identifies the serving cell of the first UEdevice. The D2D request 118 is a request for D2D communicationresources. The serving cell of the first UE device 106 may be conveyedin the request 118 by including the Cell ID of the serving cell 110 ofthe first UE device 106. For the examples herein, the D2D request 118also includes information identifying the first UE device 106. Inresponse to receiving the D2D request 118, the second base station 104communicates and coordinates with the first base station 102 toestablish the D2D resources that will be assigned for D2D communicationbetween the first UE device and the second UE device. The base stationscommunicate over the backhaul 114 using X2 messaging. For the examplesherein, the first UE device 106 also sends a D2D request message to itsserving base station (the first base station 102) wherein the requestidentifies the serving cell of the second UE device 108. Accordingly,the first UE device sends a D2D request to the first base station 102including Cell ID 2. Although various techniques may be used toestablish the D2D communication resources, an example of suitabletechnique includes a procedure where the base stations bid for D2Dresources.

FIG. 2A and FIG. 2B are block diagram of D2D subframes 200, 220 thatidentify the serving cell of the UE device transmitting the D2Dsubframe. For the example, each subframe 200, 220, conveys a cellidentifier 202. Accordingly, the D2D subframes 200, 220 are examples ofthe D2D discovery signal 116 of FIG. 1. The D2D subframes, 200, 220include DMRS symbols 204 and D2D message symbols 206. Each subframe 200,220 also includes a gap 208. The D2D subframes 200, 220 include a cellidentifier 202 where the cell identifier 202 is any information thatallows other UE devices to identify the serving cell of the UE devicetransmitting the D2D subframe 200, 220. In systems operating inaccordance with a 3GPP LTE specification, the Cell ID is preferably aCell Global Identity (CGI) for 3G and eCGI for 4G systems. The CGI isuseful since inter-PLMN discovery is supported in D2D and the PLMNinformation is included in the CGI (and eCGI). In some circumstances,the Cell ID may be the Physical Cell ID (PCI). The cell identifier maybe transmitted as a message payload or may be embedded in a referencesignal (DMRS).

FIG. 2A is a block diagram of a D2D subframe 200 where the cellidentifier (e.g., Cell ID) 202 is contained in the D2D message symbols206. At least some of the D2D message symbols 210 are used to convey theCell ID. FIG. 2A, shows some D2D message symbols 212 that are not usedfor the cell identifier 202. In some situations, all the D2D messagesymbols can be used for the cell ID 202. As mentioned above, the D2Dsubframe 200 may be a D2D discovery subframe in some situations.Accordingly, the cell identifier 202 may be transmitted in the D2Ddiscovery message of a D2D discovery signal.

FIG. 2B is a block diagram of the D2D subframe 220 where the cellidentifier 202 is embedded in the reference signal. The reference signal(DMRS) of the D2D subframe 220 is manipulated to reflect the cellidentifier 202. The reference signal, for example, may be scrambled witha code that is correlated to, or otherwise reflective of, the Cell IDsuch that another UE device can retrieve the Cell ID when decoding theD2D discovery subframe 220. Therefore, the scrambling sequence forscrambling the DMRS of the D2D subframe is a function of the cellidentifier 202. The scrambling sequence may also be a function of theD2D group ID and/or the UE device ID. The D2D message symbols 206 can beused for purposes other than the cell identifier 202. The D2D subframe220 may be a D2D discovery subframe in some situations. In suchsituations, the scrambling sequence for scrambling the RS of the D2Ddiscovery signal is a function of the cell identifier.

FIG. 3 is a block diagram of a wireless user equipment (UE) device 500suitable for use as the UE devices 106, 108. Each of the UE devicesincludes a transceiver 302 a controller 304, as well as other componentsand circuitry (not shown) such as memory and a user interface, forexample. The transceiver 302 includes a transmitter 304 and a receiver306. The transceiver 302 transmits uplink wireless signals to basestations and receives downlink wireless signals from the base stations.The transceiver can also be configured to transmit and receive D2Dsignals using allocated uplink communication resources. The controller304 controls components of the mobile wireless communication device tomanage the functions of the device described herein as well as tofacilitate the overall functionality of the device 300. The controller304 is connected to the transceiver 302 and other components such asmemory.

FIG. 4 is a block diagram of a base station 400 suitable for use as thebase stations 102, 104. Each of the base stations includes a transceiver402, a base station controller 404, and a network interface 406. In somesituations, the components may not be collocated. For example, the basestation controller 404 may be separated from the transceiver 402. Thetransceiver 402 includes a transmitter 408 and a receiver 410. Thetransceiver 402 receives uplink wireless signals from UE devices andtransmits downlink wireless signals to UE devices. The base stationcontroller 404 controls components of the base station 400 to manage thefunctions of the base station described herein as well as to facilitatethe overall functionality of the base station 400. The base station 400communicates with the network and other base stations through thenetwork interface 406. For the examples, the network interface 406 isconfigured to communicate with other base stations using X2 signalingover the backhaul 114.

FIG. 5 is a messaging diagram of an example of establishing inter-cellD2D communication. The first UE device (UE1) 106 is within the firstcell 110 served (provided) by the first base station 102 and the secondUE device (UE2) 108 is within the second cell 112 served (provided) bythe second base station 104. Accordingly, the first base station 102 isthe serving station of the first UE device 106 and the second basestation 104 is the serving station of the second UE device 108.

At transmission 502, the first UE device 106 transmits a D2D subframe116 where the D2D subframe 116 identifies the serving cell of the firstUE device 106. For the example, the D2D subframe 116 is transmittedwithin the Physical Uplink Shared Channel (PUSCH) allocated for D2Dtransmissions. The cell ID (Cell ID 1) of the first cell is transmittedwithin the D2D message or is reflected in the reference signal.

At transmission 504, the second UE device 108 transmits a D2D subframewhere the D2D subframe identifies the serving cell of the second UEdevice 108. For the example, the D2D subframe is transmitted within thePUSCH allocated for D2D transmissions. The cell ID (Cell ID 2) of thesecond cell is transmitted within the D2D message or is reflected in thereference signal.

At transmission 506, the second UE device transmits a D2D request withthe Cell ID 1 to the second base station 104. The message can be an RRCmessage such as ProSeUEInformation message. A Physical Uplink SharedChannel (PUSCH) can be used. The D2D request at least indicates arequest for D2D resources for D2D communication with the first UE device106 and identifies the serving cell of the first UE device 106. For theexample, the D2D request includes the cell ID (Cell ID 1) of the servingcell 110 of the first UE device. Other information may also be includedin the D2D request.

At transmission 508, the first UE device transmits a D2D request withthe Cell ID 2 to the first base station 102. The transmission may be anRRC message such as ProSeUEInformation message. A suitable channel forsending the message is the PUSCH. The D2D request at least indicates arequest for D2D resources for D2D communication with the second UEdevice 108 and identifies the serving cell of the second UE device 106.For the example, the D2D request includes the cell ID (Cell ID 1) of theserving cell 112 of the second UE device 108. Other information may alsobe included in the D2D request.

At event 510, the base stations coordinate to identify the D2Dcommunication resources can be allocated to the D2D UE devices for D2Dcommunication. Therefore, at transmissions 512 and 514, the first basestation communicates with the second base station to identify the D2Dcommunication resources that will be allocated for D2D communicationbetween the first UE device and the second UE device. The coordinationmay take several transmissions between the base stations.

At transmission 516, the first base station sends D2D communicationresource information (CRI) to the first UE device. The D2D CRI can bebroadcast or provided via dedicated signaling. For example, the D2D CRIcan be sent using the RRCConnectionReconfiguration message and includingthe proseCommConfig Information Element in the message. A SIB18 or SIB19message can also be used. A suitable channel for the CRI is the PDSCH.

At transmission 518, the second base station sends D2D communicationresource information (CRI) to the second UE device.

At event 520, the first UE device and the second UE device communicateover a D2D communication link using the assigned D2D resources.

The messaging discussed with reference to FIG. 5 provides an example ofutilizing cell identification information that is received in a D2Dsubframe (such a D2D discovery subframe) transmitted from a UE devicewithin a different cell. The cell identification information may be usedin other ways in some situations. For example, instead of sending arequest for D2D resources for D2D communication at transmission 506 andtransmission 508, one or both of the UE devices can send request forresources for D2D discovery signal transmission to their serving basestations where the request includes the Cell ID of the serving cell ofthe other UE device. The base stations 102, 104 can then coordinate toestablish appropriate D2D discovery signal resources. In anothersituation, the UE devices 106, 108 may report the cell ID without makinga request for resources. Such a scenario allows the base stations tocoordinate and reserve adequate D2D resources for future D2Dtransmissions although no D2D resources requests were made at the timethe cell ID was reported. Therefore, such a scenario may be useful inreducing delays for assigning D2D resources due to the time it takes tocoordinate between the base stations. The UE devices may use aProSeUEInformation message to request D2D discovery signal resourcesand/or to report the cell ID.

In some situations, a D2D UE device may transition from one cell toanother. The D2D UE transitioning to a new cell cannot continue to usethe same D2D resources since those resources may not be authorized foruse by the D2D UE device in the new cell. As a result, a procedure isexecuted to transition the D2D UE device to the new cell and continueD2D communication in the inter-cell D2D arrangement.

FIG. 6A, FIG. 6B and FIG. 6C are illustrations of the communicationsystem 100 as the second D2D UE device 108 moves from the first cell 110to the second cell 112. In FIG. 6A, the first D2D UE device 106 and thesecond UE device 108 communicate using D2D while both UE devices 106,108 are within the first cell 110. In FIG. 6B, the second UE device 108is moving from the first cell 110 to the second cell 112 and is engagedin a handover from the first cell 110 to the second cell 112. Duringthis cell transition and handover, the UE devices cease D2Dcommunication. When the handover is completed, the second UE device 108transmits a D2D subframe such as D2D discovery signal identifying itsnew serving cell.

In FIG. 6C, the handover is complete and the second UE device transmitsthe D2D subframe 602 with the second cell identifier (Cell ID 2). Asdiscussed above, the Cell ID may be transmitted within the D2D messageor may be embedded in the reference signal. The D2D subframe may be aD2D discovery subframe. The second UE device 108 also sends a D2Drequest 604 to the second base station 104.

The first UE device 106 reports the new serving cell (cell 2) of thesecond UE device 108 to the serving cell (cell 1). For the example, thefirst UE device 106 transmits a D2D Cell ID update message 606 to thefirst base station 102. The D2D Cell ID update message 606 includes theCell ID of the new cell (second cell) serving the second UE device andthe UE ID of the second UE device 108. For inter-frequency situations,the D2D Cell ID update message 606 also includes the channel frequencynumber. For example, the D2D Cell ID update message 606 includes thechannel frequency number identifying serving frequency of the secondcell. An example of a suitable structure and transmission of the D2DCell ID update message 606 includes using structure in accordance with a3GPP LTE Rel-08 message structure and transmitting the message over thePUSCH/PUCCH. For example, 3GPP LTE standards specify aProseUEInformation message that can be sent to its serving cell if itsserving cell broadcasts SIB19. The broadcast of SIB19 is an indicationto the UE that the serving cell supports D2D discovery. Similarly, ifthe serving cell broadcasts SIB18, it is an indication to the UE thatthe serving cell supports D2D communication. Currently, theProseUEInformation message is used by the UE devices to indicate to thebase station the UE device's interest in D2D discovery or D2Dcommunication and to request corresponding D2D resources. In oneimplementation of the D2D Cell ID update message 606, theProSeUEInformation message is modified to include the Cell ID, the UEID, and the channel frequency number, where needed.

After receiving the D2D request 604 and the D2D Cell ID update message606, the base stations coordinate to identify the common D2D resources.Although the coordination between base stations is critical for theintra-frequency scenario due to potential interference, it is alsonecessary in the inter-frequency scenario. Even if the BS1 102 and BS2104 operate on different frequencies for D2D, typically a base station(eNB) operates on more than one frequency. For example, BS1 102 mayoperate on frequencies F1, F2 and F3, but only F1 is used for D2D, whileBS2 104 may operate on frequencies F2, F3 and F4, but only F2 is usedfor D2D. At first glance, it may be assumed that since F1 and F2 (thetwo D2D frequencies for BS1 and BS2, respectively) are operating ondifferent frequencies, no resource coordination is needed. However, BS1102 also uses F2 for cellular communication, so the D2D resources usedby BS2 104 under F2 could potentially interfere with the cellularresources used by BS1 102. The base stations broadcast the D2Dcommunication resource information within their respective cells and theD2D UE devices resume D2D communication using those D2D resources.

FIG. 7 is a message diagram where the second UE device 108 iscommunicating with the first UE device 106 and moves from the first cell110 to the second cell 112.

At event 702, the first UE device 106 and the second UE device arecommunicating over a D2D link. Both devices are in the first cell 110and being served by the first base station 102.

At transmission 704, a measurement report is transmitted from the secondUE device 108 to the first base station 102. For the example, the firstUE device is requested by its serving cell (first base station) toperform measurement of a target cell belonging to another frequency.When the configured Event Trigger (e.g., Event A5 (PCell becomes worsethan threshold1 and neighbor becomes better than threshold2)) themeasurement report is sent to the serving cell. The measurement reportmay be considered to be a handover trigger.

After the second UE device sends the measurement report 704 to the firstbase station 102, the first UE device will need to know the target cell(and target cell's frequency) in order for the first UE to be able tomonitor discovery pool belonging to the target cell. There are multipleways that for the first UE to know where to monitor this discoveryresource pool of the target cell. In one implementation, a transmission706 is transmitted by the second UE device 108 to inform the first UEdevice of the new cell ID and frequency of the second cell (targetcell). In some cases the second UE device may alternatively sendtransmission 706 just before the measurement report 704 is sent to thefirst base station 102. For example, if a measurement event is triggered(e.g., Event A5), the second UE device may send transmission 706 beforesending the measurement report 704 to the first base station 102. Inthis alternative, the handover trigger is based on the triggering of themeasurement event rather than the sending of the measurement report 704.Therefore, before the first base station sends theRRCConnectionReconfiguration message with Mobility Information (handovermessage) to the second UE device, the second UE device informs the firstUE device of the cell ID and frequency of the target cell so that UE1may begin to monitor D2D on the target frequency. The first UE devicealso directly monitors the System Information (SIB18 or SIB19) todetermine the specific D2D resources assigned by the target cell sincethis information is not provided by its serving cell. Also, a timer isstarted after the first UE device receives the cell ID and frequencyinformation from the second UE device. If the timer expires and first UEdevice does not receive a discovery signal from second UE device(transmission 714), then the first UE device determines that the secondUE device's handover was either not initiated or there was a handoverfailure. Transmission 706 is performed in inter-frequency situationswhere the frequency of the second cell is different from the frequencyof the first cell. In intra-frequency situations where the frequenciesare the same, transmission 706 is omitted.

In another implementation, transmission 706, consist only of a 1-bithandover indicator to inform the first UE device that the second UEdevice is about to be handovered to another cell, but no explicit cellID or frequency is indicated in message 706. The reference signal (RS)in the D2D subframe (such as a discovery signal subframe) is scrambledwith the cell ID of its serving cell. As a result, therefore, theinformation about the serving cell is implicitly provided in the D2Dsubframe (discovery signal). Once the first UE device receives the 1-bithandover indicator in transmission 706, it will trigger the first UEdevice to monitor discovery resource pools for all the neighbordiscovery frequencies. Upon successful handover to BS2, the second UEdevice sends a D2D subframe (discovery signal) with the Cell IDimplicitly provided in the D2D subframe such as the discovery signal(i.e. scrambled in the RS of the discovery signal). The reception of 714with the implicit Cell ID will be an indication to the first UE devicethat the handover is successful and that the timer can be stopped.

In the case of handover failure, the second UE device will attempt tore-establish on the first cell, and if the re-establishment to the firstcell is successful D2D communication may resume between the first UEdevice and the second UE device within the first cell. However, ifre-establishment fails, the second D2D UE may no longer be served by anycell; in this case the first D2D UE should not need to monitor thediscovery resource provided by the target cell in the target frequency.The timer expiry would ensure that the first D2D UE will not need tomonitor D2D resources from the target frequency.

In yet another implementation, the transmission 706 is not sent. Thecell ID is scrambled within the RS, so that the cell ID information isimplicitly provided in the discovery signal. In this implementation,this is assumed that the first UE device is always monitoring alldiscovery resource pools for all frequencies continuously regardless ifthe second UE device is undergoing handover or not.

At transmission 708, an RRCConnectionReconfiguration message withMobility Information is transmitted from the first base station to thesecond UE device. The message is consistent with conventional handoverprocedures and initiates the handover from the serving cell. T304 is atimer that is started when the second UE device receives theRRCConnectionReconfiguration message. This timer is used in case ofhandover failure and is not related to D2D operation.

At transmission 710, a handover completion message is sent form thesecond UE device to the second base station. The T304 timer is stoppedafter the message is sent.

At transmission 712, the second UE device sends an RRC message (e.g.,ProSeUEInformation message) to request D2D transmit resources from thesecond base station (target cell).

At transmission 714, the second UE device transmits a D2D subframe suchas D2D discovery signal to the first UE device. The D2D subframe (D2Ddiscovery signal) includes the Cell ID of the second cell. The first UEdevice is monitoring the frequency of this cell either because it is anintra-frequency situation or it is an inter-frequency situation and thefirst UE device was informed of the target frequency at transmission706. For inter-frequency scenarios, the reception of this D2D message isan indication that the handover is completed successfully for the secondUE device and the first UE device should continue to monitor discoveryresources belonging to the target cell in the target frequency. Thetimer started in transmission 706 is stopped. As explained above, if thetimer expires (not stopped), the first UE device determines that thehandover for the second UE device was not successful and the first UEdevice stops monitoring the target cell/frequency. In the case ofhandover failure, the second UE device will attempt to re-establish onthe first cell, and if the re-establishment to the first cell issuccessful D2D communication may resume between the first UE device andthe second UE device within the first cell. However, if re-establishmentfails, the second D2D UE may no longer be served by any cell; in thiscase the first D2D UE should not need to monitor the discovery resourceprovided by the target cell in the target frequency. The timer expirywould ensure that the first D2D UE will not need to monitor D2Dresources from the target frequency.

The D2D subframe is transmitted as soon as possible after the handoveris complete. Therefore, where the D2D subframe is a discovery subframe,the D2D discovery signal is transmitted in response to completion of thehandover and is sent independent of a status of a discovery signalbuffer at the D2D UE device. In accordance with the 3GPP specifications,the UE devices can only transmit discovery signals when configured(assigned) a communication resource for transmission. Typically, asemi-persistent transmission resource is scheduled to the UE if the UEis interested in sending a D2D discovery signal. The length of thesemi-persistence resource may be dependent of the buffer size of theintended D2D discovery signal. However, if the UE device is notinterested in transmitting any discovery signal no resources may berequested by the UE device. Therefore, there may be a period of timewhen the UE device does not send any D2D discovery signals. Accordingly,after the handover is initiated 704, the second UE device transmits D2Ddiscovery signals in accordance with the schedule provided by the firstbase station. However, if the second UE device does not have D2Ddiscovery resource, it sends a request for such resources from itssource base station (first base station) before sending the measurementreport 704. Alternatively, the second UE device may use the commondiscovery resource allocated over the SIB19 or (SIB18 in case of D2Dcommunication) by the first base station for D2D discovery transmission.The D2D transmissions are suspended during the handover includingsuspension of D2D discovery signal transmission. When the handover iscompleted, the second UE device must be configured by the second basestation with the resource for transmitting the D2D discovery signal orthe second UE device may be configured to use common discovery resourceallocated over SIB19 by the second base station (target base station).The second UE device should transmit a discovery subframe to second UEdevice even if the second UE device is not interested in transmittingD2D discovery signal (e.g., D2D data buffer is empty). This discoverysubframe 714 must be received by the first UE device in order for thefirst UE device to stop the timer.

At transmission 716, a cell ID update message is sent from the first UEdevice to the first base station. The message informs the serving cellof the first UE device that the second UE device has the second cell(Cell ID 2) as a serving cell.

At event 718, the base stations coordinate to determine common resourcepools based on priority (if needed). Accordingly, the base stationsidentify the D2D resources that can be used by the UE devices 106, 108for D2D communication.

At transmission 720, the first base station provides the updatedresource pool information (the communication resources information(CRI)) to the first UE device by dedicated signaling or by broadcastingthe information in SIB19.

At transmission 722, the second base station provides the updatedresource pool information (the communication resources information(CRI)) to the second UE device by dedicated signaling or by broadcastingthe information in SIB19.

At event 724, the UE devices 106, 108 resume D2D communication using theresources for the pool of resources provided by the base stations.

Clearly, other embodiments and modifications of this invention willoccur readily to those of ordinary skill in the art in view of theseteachings. The above description is illustrative and not restrictive.This invention is to be limited only by the following claims, whichinclude all such embodiments and modifications when viewed inconjunction with the above specification and accompanying drawings. Thescope of the invention should, therefore, be determined not withreference to the above description, but instead should be determinedwith reference to the appended claims along with their full scope ofequivalents.

What is claimed is:
 1. A method comprising: serving, by a first basestation, a first device-to-device (D2D) user equipment (UE) device and asecond D2D UE device; initiating a handover of the second D2D UE deviceto a second base station; refraining from performing D2D communicationbetween the first D2D UE device and the second D2D UE device during thehandover; receiving, at the first D2D UE device, a D2D subframe from thesecond D2D UE device, the first D2D UE device within a first cellprovided by the first base station and the second D2D UE device within asecond cell provided by the second base station, the D2D subframeconveying a cell identifier identifying the second cell, the D2Dsubframe indicating that the handover is complete and that the first D2DUE device is permitted to attempt to resume D2D communication with thesecond D2D UE device; and sending, to the first base station, a requestfor D2D communication resources for D2D communication between the firstUE device and the second D2D UE device, the request conveying the cellidentifier identifying the second cell to the first base station.
 2. Themethod of claim 1, wherein the D2D subframe includes a D2D messagecomprising the cell identifier.
 3. The method of claim 1, wherein ascrambling sequence for scrambling a reference signal of the D2Dsubframe is a function of the cell identifier.
 4. The method of claim 3,wherein the scrambling sequence is a function of a device identifier ofthe second D2D UE device.
 5. The method of claim 3, wherein thescrambling sequence is a function of a D2D group identifier of a D2Dgroup comprising the first D2D EU device and the second D2D UE device.6. The method of claim 1, further comprising determining whether thesecond D2D UE device is an intended receiver of D2D transmissions fromthe first D2D UE device and wherein the request conveys the cellidentifier to the first base station when the second D2D UE device isthe intended receiver.
 7. The method of claim 1, wherein the handover istriggered based on a measurement event triggered within the D2D UEdevice.
 8. The method of claim 1, wherein the D2D subframe is a D2Ddiscovery subframe.
 9. A method performed at a first base station, themethod comprising: serving a first device-to-device (D2D) user equipment(UE) device and a second D2D UE device; handing over the second D2D UEdevice to a second base station; receiving, from the first D2D UE devicewithin a first cell provided by the first base station, a request fordevice-to-device (D2D) communication resources for D2D communicationbetween the first D2D UE device and at least the second D2D UE device,the second D2D UE device within a second cell provided by a second basestation, the request comprising a cell identifier of the second cell,the request sent in response to the first D2D UE device receiving a D2Dsubframe indicating that the handing over of the second D2D UE device iscomplete and that the first D2D UE device is permitted to attempt toresume D2D communication with the second D2D UE device, which was ceasedduring the handing over; and communicating with the second base stationto identify common D2D communication resources for the D2Dcommunication; and assigning the common D2D communication resources tothe first D2D UE device for the D2D communication.
 10. The method ofclaim 9, wherein the cell identifier is determined by the first D2D UEdevice from a D2D subframe transmitted by the second D2D UE device. 11.The method of claim 10, wherein the D2D subframe includes a D2D messagecomprising the cell identifier.
 12. The method of claim 10, wherein ascrambling sequence for scrambling a reference signal of the D2Dsubframe is a function of the cell identifier.
 13. The method of claim12, wherein the scrambling sequence is a function of a device identifierof the second D2D UE device.
 14. The method of claim 12, wherein thescrambling sequence is a function of a D2D group identifier of a D2Dgroup comprising the first D2D UE device and the second D2D UE device.15. The method of claim 9, wherein the D2D subframe is a D2D discoverysubframe.
 16. A method comprising: handing over a first device-to-device(D2D) user equipment (UE) device from a first base station to a secondbase station; refraining from performing D2D communication between thefirst D2D UE device and a second D2D UE device during the handing over;generating, at the first D2D UE device within a first cell provided bythe second base station, a D2D subframe; and transmitting the D2Dsubframe from the first D2D UE device to the second D2D UE device withina second cell provided by the first base station, the D2D subframeconveying a cell identifier identifying the first cell to the second D2DUE device, the D2D subframe indicating that the handing over is completeand that the second D2D UE device is permitted to attempt to resume D2Dcommunication with the first D2D UE device.
 17. The method of claim 16,wherein the D2D subframe includes a D2D message comprising the cellidentifier.
 18. The method of claim 16, wherein a scrambling sequence ofa reference signal of the D2D subframe is a function of the cellidentifier.
 19. The method of claim 18, wherein the scrambling sequenceis a function of a device identifier of the first D2D UE device.
 20. Themethod of claim 19, wherein the scrambling sequence is a function of aD2D group identifier of a D2D group comprising the first D2D UE deviceand the second D2D UE device.
 21. The method of claim 16, wherein theD2D subframe is a D2D discovery subframe.
 22. A device-to-device (D2D)user equipment (UE) device comprising: a receiver configured to: receivean indication to refrain from performing D2D communication between theD2D UE device and another D2D UE device during handover of the anotherD2D UE device to a neighbor cell different from a serving cell of theD2D UE device, and receive a D2D subframe from the another D2D UE devicelocated in the neighbor cell, the D2D subframe conveying a cellidentifier identifying the neighbor cell, the D2D subframe indicatingthat the handover is complete and that the D2D UE device is permitted toattempt to resume D2D communication with the another D2D UE device; anda transmitter configured to transmit to a serving base station providingthe serving cell, a request for D2D communication resources for D2Dcommunication between the D2D UE device and the another D2D UE device,the request conveying the cell identifier to the serving base station.23. The D2D UE device of claim 22, wherein the D2D subframe includes aD2D message comprising the cell identifier.
 24. The D2D UE device ofclaim 22, wherein a scrambling sequence for scrambling a referencesignal of the D2D subframe is a function of the cell identifier.
 25. TheD2D UE device of claim 24, wherein the scrambling sequence is a functionof a device identifier of the second UE device.
 26. The D2D UE device ofclaim 22 further comprising: a controller configured to determinewhether the another D2D UE device is an intended receiver of D2Dtransmissions from D2D UE device and wherein the request conveys thecell identifier to the serving base station when the another UE deviceis the intended receiver.
 27. The D2D UE device of claim 22, wherein theD2D subframe is a D2D discovery subframe.